This invention relates to telephone extension systems providing a remote transmitting and/or receiving device which communicates over AC power wires with a subscriber's telephone line. More particularly, the present invention provides apparatus for communicating over AC power wires between a remote location and a conventional telephone line.
Heretofore , it has been proposed to provide a remote device (an extension telephone) communicating with a conventional on line telephone through transmission means other than a conventional extending telephone line. Various wired and wireless telephone extension systems have been proposed. The wireless systems consist of transmitting and receiving radios at the on line telephone and at the extension telephone. These systems encounter unique problems relating to the power and frequency of radiated signals as well as other problems of signal loss between the on line and extension phones due to physical shielding between them. Furthermore, since all signals between the on line and extension telephones are transmitted, they can be intercepted and interferred with freely by anyone within range.
The wired telephone extension systems that have been proposed make use of the AC power lines for communicating between the on line and the extension telephones. In these, the subscriber's on line telephone which is connected directly to the telephone line, is specially equipped with an acoustical microphone and speaker that communicate audio signals to the on line telephone and the acoustical devices are contained in an auxiliary unit that goes alongside the on line telephone and holds the telephone handset. These acoustical devices, in turn, communicate electrically with an extension telephone, through the available AC power wires and when this system is in use, an incoming telephone call can be answered at the extension telephone as well as at the on line telephone. However, the incoming call cannot be answered at both the on line and the extension telephones, because when the on line handset is removed from the auxiliary unit, the acoustical couplings are broken and then the extension telephone is incapable of either receiving or transmitting to the on line telephone. Furthermore, the acoustical devices at the auxiliary unit will pick up surrounding noise as well as the voice signals and this noise will be transmitted along with the voice signals to the telephone line or over the AC power line to the extension telephone.